Today there is a wide diversity of small-size mobile photo devices which are either detachable or built into other appliances such as cellular telephones which allow for taking pictures of various objects. High-aperture optics for low light conditions cannot be used in such devices because of the strict size and weight limitations.
There are two well known ways of obtaining pictures in low light conditions when high-aperture optics are not applied. They can be used either separately or in combination:
1. Longer exposure time, sometimes, when necessary, in combination with optical or digital image stabilizers (‘Exposure in digital photography,’ Chris Weston. ISBN 978-5-9794-0235-2, 978-2-940378-29-6).
This method has a weakness: in case of long exposure and when picture is taken with a hand-held camera which is not anchored solidly during the exposure, the resulting image is blurry, fuzzy and not sharp. Using additional mechanical devices (such as a tripod) does not meet mobility requirements. This weakness is partially compensated with a digital or optical image stabilizer. However, such devices make cameras larger. In addition, the weakness of the first method, which cannot be eliminated by the use of image stabilizers, is the fact that moving objects in the scene appear blurred.
2. Intensive amplification in the data reading path from the sensor (equivalent to increased sensitivity value) (‘Exposure in digital photography,’ Chris Weston. ISBN 978-5-9794-0235-2, 978-2-940378-29-6).
The weakness of the second method is in amplification of not only brightness of an image, but also noise from the sensor and image reading/digitalization path. There are methods to suppress noise afterwards, however, they inevitably make certain details in the image disappear, too.
There are descriptions of a procedure for receiving several (a series of) exposures from the sensor comprising the following stages (http://www.aptina.com/products/image_sensors/—data table for contemporary sensors; http://www.aptina.com/assets/downloadDocument.do?id=373—detailed description of a sensor with an electronic shutter: mt9p013):
determination of optimal exposure time,
exposure during time T1 and following data readout,
interval between exposures,
exposure during time T2 and following data readout,
interval between exposures,
exposure during time T3 and following data readout,
and so on.
The weakness of the above picture taking method is in exposures being scattered over time, which worsens the quality of display of moving objects after exposures are fused.
The method of image enhancement described in U.S. Pat. No. 450,346 is closest to proposed invention. This method suggests that two frames are used as input images and one of these images has an improved signal/noise ratio, however, with certain distortions. The method uses low-frequency data from the first image and high-frequency data from the second one. Additional logic is used to determine the threshold frequency for the two images. The frequency can change adaptively over the image region depending on the image characteristics. This method is used in the following way to reduce spurious signals in an image:                several exposures of an object are obtained;        the first exposure is taken (with a low signal/noise ratio);        several images with a high signal/noise ratio are fused into a single image producing the second image;        low-frequency filtering is used on the first image to reduce noise;        the second image undergoes high-frequency filtering;        frequencies of low and high-frequency filtering are adjusted to the extent of local distortions (artifacts);        the filtered images are combined by simple summation.        
The weakness of this method is blurriness in the image which cannot be eliminated. If there are moving objects in the scene, images taken by this method with exposures separated in time will inevitably contain artifacts.